Hydraulic systems

ABSTRACT

This invention relates to a hydraulic system for example for use in an earth moving machine, an excavator, a jib crane, or a corresponding machine for contracting activities, comprising at least one hydraulic double acting hydraulic motor, each being controlable and reversible through a control device, and a pump unit powered by a prime mover, for example a diesel engine or an electric motor, for providing the required hydraulic effect.

O r United States Patent 1151 3,678,684 S6rensen 1 1 July 25, 1972 [S4] HYDRAULIC SYSTEMS 2,3l2,2l3 2/1943 Ferris ....60/97 8 X 2,328,980 9/ I943 Herman et .60/52 R UX [721 w S My 2,400,685 5/[946 Collins .......60/97 1; [73] Assignee: Anstellt Fur Elekuohydrauilsche Anlegen, 2.640.323 6/1953 McLeod .60/52 HC Vaduz, Liechtenstein 2,984,985 5/l96l MacMillin ....60I97 F 3,018,902 l/l962 Minty ....60/97 F X m] 3,l6l,245 [2/1964 Thoma ..60li9 x [2]] Appl. No.: 76,760

Primary Examiner-Edgar W. Geoghegan 30] F I n A H on i I I I Attorney-Ernest F. Marmorek 06:. 6, 1969 Denmark ..s297/69 [511 AISTRACT This invention relates to a hydraulic system for example for [52] US. Cl. ..60Il9, 60/52 use in an an moving machine an excavator. ajib crane Ora l 5 I] In. c F02. b corresponding machine for contracting activities, comprising at can one hydraulic double an! hydraulic motor. "ch [58] Field 0! Search ..60/52 VS, 19, DIG. 2, 97 E being commlable and reversible mouth a comm device. and t 56] R I a pump unit powered by a prime mover, for example a diesel engine or an electric motor, for providing the required UNITED STATES PATENTS hydraulic R19,694 9/1935 Ernst ..60/52 HF ll Cllllm, 4 Drawing Figures 9 n1" i 51! I c 116 I20 5 (2 ll 39,; '6

V 33 a 33 c 40A 40 I Patented July 25, 1972 4 Shuts-Shoot 2 FIG 2.

INVEN TOR: [a Emil 72 an,

M/WJ

fi m

INVEVTOR A 5% 1:72am M M 4 Shuts-Shut 4.

Patented July 25, 1972 2w Em H E 3.

HYDRAULIC SYSTEMS In such hitherto known hydraulic systems a pump-unit has been used, consisting of one or more adjustable pressure compensated pumps, each having its discharge port connected, ofien through a pressure accumulator, to the cylinder or cylinders of the hydraulic motor, whereas a valve device, usually a slide valve, is inserted in the connection, by means of which valve device the flow of hydraulic fluid to the cylinder or cylinders can be directed either to the one or to the other of the inlets thereof or can be fully cut oil, and the outlet of each cylinder of the hydraulic motor if through the valve device connected to a reservoir for hydraulic fluid, to which also the suction port or ports of the pump or pumps are connected.

In a mechanical unit comprising a hydraulic system of the type concerned, the rate of movement of the piston of a hydraulic motor, in addition to being dependent on the degree of opening of the valve device, depends essentially on the forces counteracting the movements of the piston and, in case more cylinders of the hydraulic motor are connected to the same pump and are operated simultaneously, on the number of cylinders simultaneously operated by pressurized fluid from the pump, as well as on the pressure in the cylinder of the most loaded hydraulic motor.

Consequently, when a certain working speed of a machine part operated by means of a hydraulic motor is desired, the control member of the control device of the said hydraulic motor must in each individual case be adjusted to the working conditions. In practice, however, this adjustment is frequently neglected, and as a consequence the operation of such mechanical units in too many cases, proceeds at a rate essentially lower than is actually desired and, in case a number of hydraulic motors are operated by pressurized fluid from the same pump or pump unit, it will in many cases not even be possible to attain the endeavored maximum working speed of a machine part connected to a hydraulic motor.

Moreover, a control by more or less throttling the hydraulic fluid results in a reduction of the effiency of the hydraulic system.

The main object of the present invention is to provide a hydraulic system of the type concerned, by means of which it is possible, in any case within the load limits of the prime mover and the maximum capacity of the hydraulic motor, to obtain any desired working speed of the hydraulic piston, from zero to maximum speed, simply by adjusting the control device to a definite position corresponding to the desired working speed. In case the control device is adjusted by a servo-actuator, operated partly by an operating value-adjustment, adjustable for example by a handle, and by an actualvalue-adjustment actuated by the movement of the hydraulic, this may cause that the working speed will, under normal working conditions, be dependent only on the rate of movement of the operating handle of the operation-value-adjustment.

According to the present invention such a hydraulic system is obtained thereby that for each hydraulic motor the pump unit comprises an adjustable hydraulic pump connected solely to the hydraulic motor and of the type having for each adjustment a constant voluminal yield, and being in a manner known per se, by means of the control device of the corresponding hydraulic motor preferably continuously but at least stepwise adjustable from delivery of maximum positive voluminal yield to one inlet of the hydraulic motor, through a neutral position to delivery of maximum negative yield to the other inlet of the hydraulic motor.

The terms "positive voluminal yiel and negative voluminal yield are used here only with a view to distinguishing the two directions of movement of the hydraulic, since it may occur that the working speed by movement in one direction must always remain below a certain value, whereas a corresponding limitation is not necessary by movement in the opposite direction, in which case the pump connected to such a hydraulic motor may be so constructed that for example its maximum negative voluminal yield" is lower than its "maximum positive voluminal yield".

It should also be noted that a hydraulic motor will usually consist of one single jack, or of a number of interconnected jacks, but is is that the hydraulic motor for a secondary working device of a mechanical unit can include two or more mutually independent jacks, to which the hydraulic fluid is distributed by means of valve devices with fixed or adjustable setting.

A hydraulic pump of the said type will by each setting of the control device, apart from neutral, always yield the same volume of hydraulic fluid irrespective of counterpressure, and therefore each setting of the control device will secure the same constant working speed of the piston or pistons of the corresponding hydraulic motor, in any case as long as the counterpressure is not high enough to cause an overloading of the prime mover and thereby a reduction of its normally constant number of revolutions. In addition, since the control of such a pump does not cause any loss through throttling, the hydraulic system will work with an essentially higher efficiency than it has been possible by the valve controlled hydraulic systems hitherto used. A further essential advantage is that by a hydraulic system according to the present invention and comprising a number of hydraulic motors connected to a pump each, the hydraulic motors can be operated simultaneously and each with the working speed most convenient, without the operation of the individual hydraulic motor effecting the operation of the others, so that a mechanical unit provided with such a hydraulic system can be operated at an essentially higher speed than like mechanical units provided with conventional valve controlled hydraulic systems. The said possibility of simultaneous utilization of several hydraulic motors under attainment of maximum performance of each hydraulic motor, provided the prime mover is not overloaded thereby, results further therein that during essentially longer periods than hitherto possible the prime mover may operate near full capacity, and thereby with the highest possible effiency, which in turn improves the economy of the mechanical unit.

In a hydraulic system according to the invention each hydraulic pump and the hydraulic motor connected thereto will usually operate so that the hydraulic fluid leaving the hydraulic motor will fully or partly return to the hydraulic pump. Therefore, in case the hydraulic piston is exposed to considerable forces, acting in the direction of the desired movement of the piston, the hydraulic pump will function as a generator supplying energy unloading the prime mover, but since the voluminal yield of fluid supplied to the suction side of the hydraulic cylinder is constant, depending on the setting of the hydraulic pump, a vacuum may develop in the suction side providing cavitation in the hydraulic fluid, also in the part thereof to be found in the hydraulic pump, and such cavitation will not only result in a considerable noise in the pump but also, in the long run, involve too early destruction thereof.

This can be avoided thereby that the hydraulic connection between at least one of the inlets of the hydraulic motor and the corresponding port of the hydraulic pump comprises a valve device adapted to throttle the flow of hydraulic fluid in direction away from the hydraulic motor, so that a counterpressure is developed on the pressure side of the hydraulic cylinder of such'a' magnitude that no cavitation producing vacuum develop on the suction side of the hydraulic cylinder.

Another advantage is that such a throttling valve device will fully or partly absorb and transform into heat the potential or kinetic energy of the piston rod of the hydraulic motor and the devices connected thereto, so that there is no need of otherwise frequently required brake devices, involving additional cost of the hydraulic systems and comprising devices prevent ing the prime mover from racing.

As aforementioned it is an advantage of the hydraulic system that it permits operation at approximately full load of the prime mover.

This involves also, however, that in certain situations a risk, non-existent with previous methods of control, of overloading of the prime mover may develop.

Therefore, the hydraulic system may comprise means for measuring the load of the prime mover, means for measuring the total output of the hydraulic pumps, means for comparison of said load and the said output, and control-actuating devices adapted to actuate the control devices of the hydraulic pumps, whenever said total output exceeds the output corresponding to full load of the prime mover. In this manner it becomes possible to secure that the output of all pumps involved and, in turn, the effect is reduced in case the prime mover being over loaded, so that impermissible overloading can be avoided.

The prime motor of hydraulic systems consists often of an internal combustion engine equipped with a governor depending on the load and having a slidable or rotatable fuel control rod able to take up one position corresponding to idling without load; one position corresponding to full load at maximum number of revolutions; and one position corresponding to full load at minimum number of revolutions, that is maximum overload, and in such cases the control devices of each pump of the hydraulic system is connected to and adapted to be adjusted by a corresponding electrically controlled servoactuator.

ln a hydraulic system according to the invention and of this type it may be appropriate that the control rod is adapted to operate, at least during its movements between the position corresponding to full load at maximum number of revolutions and its position corresponding to full load at minimum number of revolutions, to move the slidable tap of an adjustable electric resistance, for example a potentiometer, in such a manner that the voltage over an electric safety circuit, fed through the resistance and containing one or more actuating devices for actuation of a control device in the feed circuit of each individual servo-actuator, is changed so that the yield of the hydraulic pump corresponding to each servo-actuator is reduced during the said movement of the control rod. A govenor as the one referred to tends, where the engine is loaded up to nominal power, to maintain a constant number of revolutions and may be said to be a device, by means of which the effect produced by the engine, corresponding to the effect transmitted through the hydraulic pumps, is compared with the load of the prime mover, that is with the adjustment of the fuel supply system, while the control rod is at any time adjusted to the result of the said comparison. Thus, such a control device as described will not be actuated until incipient overloading develops, in which case, however, the yield of the hydraulic pumps immediately declines and the more the higher the incipient overloading is, whereby an overload involving stoppage of the prime mover is avoided.

The foregoing and other objects and advantages of the present invention will be appreciated upon reading the following description in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram of an embodiment of the hydraulic system adapted to actuate three hydraulic motors in the form of hydraulic jacks;

FIG. 2 is a diagram of another embodiment of the hydraulic system comprising, however, for the sake of clarity, only one single jack, the different pressure conditions in the pipes being indicated by the use of lines of different thickness for the pipes;

FIG. 3 is a schematical block diagram, illustrating the electric devices of the hydraulic system adapted to prevent over loading; and

FIG. 4 in a similar manner another embodiment of a system for preventing overloading.

The hydraulic system illustrated in FIG. 1 is provided with three hydraulic jacks, WA, 108 and C.

On the drawing the reference numbers indicating parts of the hydraulic systems described in the following, belonging to each of these jacks 10A, 10B, and 10C, are supplemented by letters corresponding to the jack in question, but for simplicity, however, in the following descriptions such parts are referred to only through the reference number proper when the reference does not apply to one particular jack out of the three, or to the hydraulic system connected thereto.

Each jack comprises a cylinder 11, a piston 12 reciprocably arranged therein, and a piston rod 13 connected to the piston 12. Each cylinder 11 has two inlets, an inlet 14 opening into the part of the cylinder opposite to the free end of the piston 12, and an inlet 15 opening into the part of the cylinder, through which the piston rod 13 is passing.

The hydraulic systems for the three jacks comprises a common hydraulic fluid reservoir 16 communicating with a return pipe 17 provided with a non-return valve 18 and a filter 19, and with a suction pipe 20. The two pipes 17 and 20 are interconnected through a non-return valve 21 preventing passage of fluid from the return pipe I! to the suction pipe 20. The three hydraulic systems comprise, moreover, a common safety return pipe 22 which, through a relief valve 23, is connected to the return pipe 17.

The hydraulic system for each jack 10 includes a hydraulic pump 24 with two ports 25 and 26. The three pumps 24A, 24B and 24C are driven by a common shaft 27 powered by a prime mover indicated by a circular arrow 28. On the drawing, for simplicity, the shaft 27 and the prime mover 28 are shown in connection with each of the pumps 24, although they are used for all of the pumps jointly.

Each pump 24 is preferably a variable volume reversible swash plate type pump. The adjustment of the angular position of the swash plate is affected by means of a servo-actuator 29, the action of which, in a manner known per se, depends partly on a manually operated operating-valueadjustment, and partly on an actual-value-adjustment depending on the position of the corresponding piston 12 in the cylinder 11.

The pump port 25 is by means of a connecting pipe 30 comprising a flexible hose and a pilot check valve 40 connected to the inlet 14 of the jack 10, whereas the pump port 26 through a connecting pipe 32 with a flexible hose 33 is connected through a pilot check valve 39 to the inlet 15 of the jack [0. The two connecting pipes 30 and 32 are, through non-return valves, 34 and 35 respectively, connected to the safety return pipe 22, each of these non-return valves permitting only transfer of hydraulic fluid from the corresponding connecting pipe 30 or 32 to the safety return pipe, but only in case the pressure in the connecting pipe rises above permissible limits, the safety pressure being controlled by the relief valve 23.

The connecting pipe 32 is moreover connected to the suction pipe 20 through a non-return valve 36 permitting passage of fluid only from the suction pipe 20 to the connecting pipe 32. The connecting pipe 30 is connected to the return pipe [7 though a pilot controlled check valve 37 normally preventing passage of fluid from the connecting pipe 30 to the return pipe 17 but controlled by the pressure in the connecting pipe 32, and so that it opens when the pressure in the pipe 32 exceeds a certain value. This arrangement in connection with the connecting pipes 30 and 32 being connected to the reservoir 16 is due to the circumstance that when the pump supplies pressure to the pump port 24 and the piston 12 thereby moves downwards on the drawing the amount of fluid leaving the inlet 14 of the cylinder 10 will exceed the amount supplied to the jack cylinder 11 through the inlet 15 and thus an amount of fluid exceeding the suction capacity of the pump 24, for which reason the surplus must be returned to the fluid reservoir 16, while, conversely, in the case of the pump supplying pressure to its port 25, more fluid must be supplied to the pump than is given ofi from the inlet 15 of the cylinder ll, so that the non-return valve 36 must permit supplementary fluid to be sucked in.

The pump 24 may, if required, be provided with a relief valve, functioning as a safety valve, which through a pipe 38 is connected to the return pipe 17.

Considering the working cylinder 10A with appurtenant hydraulic system it is seen that, if it is desired that the piston 12A shall be moved upwards following an incline on the drawing and thereby involve a corresponding movement of a mechanical component connected to the piston rod 13A the pump 24A must be so adjusted that it supplies fluid to the port 25A, whereby the swash plate of the pump 24A is given an inclination corresponding to the speed at which the movement is required to be effected. Since the pump 24A is a multicylinder piston pump, and assuming that the shaft 27 is rotated at a constant number of revolutions, it is easily seen that the rate at which the piston lZA moves upwards depends solely on the voluminal yield of the pump 24A, which, in turn, depends on the position to which the swash plate is adjusted.

The hydraulic fluid leaving the inlet 15A of the cylinder llA flows through the connecting pipe 32 back to the port 26A of the pump 24A, functioning as suction side, but since the amount of fluid leaving cylinder 11A will be less than the amount supplied through the cylinder inlet 14A the fluid leaving the cylinder "A must be supplemented 14A, fluid sucked in through the non-return valve 36A.

Assuming now that the mechanical component operated by means of the piston rod 13A, during the said movement thereof, as a consequence of a load on the said component excerts a tensile force on the piston rod 13A, this may involve that the piston tends to move upwards at a rate higher than corresponding to the amount of fluid introduced by the pump through the inlet MA, whereby may develop cavitations in the hydraulic fluid at the pressure side, which cavitations have destructive effect on the pump in the long run. For prevention hereof the pilot controlled check valve 39A is introduced into the connection between the cylinder-inlet 15A and the connecting pipe 32A so controlled by the pressure in the connecting pipe 30A that it does not open and permit outlet through the cylinder inlet 15A, until the pump has built up a certain pressure in the connecting pipe 30A, wherealter the degree of opening of the valve 39A depends on this pressure. This involves that the discharge from the cylinder 1 1A is throttled to such an extent that the movement of piston 12A is always controlled by the hydraulic fluid flowing to inlet MA from the pump 24A.

When the piston 12A has reached the position corresponding to the desired position of the mechanical component connected thereto, the swash plate of pump 24A is adjusted to neutral by means of the servoactuator 29A; when the pump 24A is in its neutral position, it acts as a closed valve for the hydraulic fluid within the cylinder so that the fluid on both sides of the piston 12A becomes enclosed in such a manner that the piston cannot move unintentionally.

A corresponding prevention of the piston 12A being driven from the mechanical component towards the inlet MA is attained by means of a pilot controlled check valve 40A inserted between the inlet 14A and the connection 30A and controlled by the pressure in the connecting pipe 32A.

As to the jack 108, this is assumed to be connected to a mechanical component that can exert a motive power on the piston 128 during its inward movement only, wherefore, in connection with the jack 103, only one check valve 403 corresponding to the check valve 40A is used, inserted between the cylinder inlet 14B and the connecting pipe 303. Conversely, the jack C is assumed to be connected to a mechanical component that can exert a force acting on piston 12C when the piston 12C is moving in the direction of the cylinder inlet ISC, wherefore, also in connection with this jack 10C only one check valve 39C corresponding to the pilot controlled check valve 39A is used, and this valve 39C is inserted between the cylinder inlet 15C and the connecting pipe 32C and controlled by the pressure in the connecting pipe 31C.

FIG. 2 shows schematically another embodiment of the present hydraulic system, but only one single jack with corresponding hydraulic system is shown, although a number of jacks may be included. This hydraulic system includes, indicated by the supplementary letter D, elements corresponding to the elements 10 to 16, 24 to 26, 30 and 32 explained in connection with FIG. 1. The hydraulic pump MD is also in this embodiment controlled through a servoactuator, which is not indicated.

In this hydraulic system a feeder pipe 50, common to a number of like hydraulic systems, in connected to the hydraulic fluid reservoir 16D. Between the pump ports 25D and 26D and the corresponding connecting pipes, 30D and 32D respectively, discharge valves 51 and 52 respectively are inserted, opening when pressure is applied to the corresponding pump port. The pipe part between the pump port 250 or 26D and the corresponding non-return valve 5] or 52 respectively are through suction valves 53 and 54 respectively connected to a pipe 55, which, in turn, through a pipe 56 is connected to the common pipe 50. The two connecting pipes 30D and 320 are directly connected to the inlets 14D and 15D respectively of the jack 10D. Between the connecting pipe 30D and the pipe 55 a pilot controlled throttle valve 58 is inserted, being opened by the pressure in the part of the connecting pipe 32 arranged between the pump port 260 and the non-return valve 52. Hence, the throttle valve 58 can be said to be inserted in a shunt inserted between the inlet MD of the jack 10D and the suction valve 53.

A corresponding pilot controlled throttle valve 60 is inserted in a shunt 61 between the inlet 15D of the jack 10D and the suction valve 54 and controlled by the pressure in the connection between the pump port 25D and the discharge valve 51.

A more detailed explanation of the control of the jack 10D by means of the hydraulic pump 24D should not be necessary, considering the explanation given above in connection with FIG. 1.

It has to be noted, however, that in case the mechanical element connected to the piston rod exerts a powerful inclined downwards pressure on the piston 12D, see FIG. 2, a high pressure will develop in the connecting pipe 30D, wherefore this pipe is shown in a particularly heavy line. However, as a consequence of the discharge valve 51, this pressure is prevented from being transferred directly to the pump port 25D and, assuming the hydraulic pump 24D is in neutral position, the pilot controlled throttle valve 58 will be closed, and, therefore, the piston 12D will be retained in the position reached. When, thereafier, the piston 12D has to be moved in the direction of the pressure, that is retracted, the pump is ad justed to supply pressure to the pump port 26D, and through the discharge valve 52, the connecting pipe 32D, and the cylinder inlet 15D to the space above the piston 12D. Since, due to the valves 51 and 58, this piston 12D is prevented from moving, the pressure in the connecting pipe 32D will rise quickly, whereby the throttle valve 58 is opened, and hydraulic fluid from the space in the cylinder llD below the piston 12D can throttle through the throttle valve, flow into the shunt pipe 59 and through the suction valve 53 to the port 25D of the pump 24D. Surplus hydraulic fluid, if any, flows through the pipe 56 and the common pipe connection 50 back to the reservoir 16D. The pipe 56 may be doubled, if necessary, in such a manner that, in one of the pipes, a suction valve is inserted permitting flow of fluid only from the pipe 50 to the pipe 55, while, in the other pipe, a relief valve is inserted that secures a certain overpressure in the shunt pipe 59, which results in the pump 24D functioning as a motor, supplying energy to the prime mover.

In the hydraulic system shown in FIG. 2 a pilot controlled throttle valve is connected to each of the connecting pipes 30D and 32D. lf, however, the piston [2D and in turn the mechanical element connected thereto can only be moved by an external force in one direction, one of the two pilot controlled throttle valves may be left out. In this case also the corresponding discharge valve 51 or 52 can be left out.

The fact that, in the present hydraulic system, each jack, or each group of interconnected jacks, is operated by means of a pump having the sole purpose of operating the cylinder, or the group of cylinders, involves, in comparison with corresponding hydraulic systems in which all jacks are operated from one single pump, an essentially higher risk of overloading of the prime mover, partly in case several jacks are being operated simultaneously with a considerable load and a high operating speed, partly in case of a single jack suddenly encountering unexpected high resistance, such as may be the case, for example, where the shovel of a power shovel hits a stone or other object difficult to move.

Such overloading can, of course, be prevented by insertion of an overload protector in the connection between the prime mover and the shaft powering the pumps; this measure will, however, involve stoppage of the machinery in its entirety.

Therefore, it is more convenient if measures by taken to secure that an overload will involve such effect on the devices controlling in any case the more essential pumps that the yield of the pumps is reduced, if necessary to zero. On the other hand, in connection with operation of a hydraulic system as described, it will be most economical to let the prime mover work as close as possible to its nominal capacity, and for this reason the invention includes an excess-load control that actuates all or in any case the more essential of the control devices of the pumps, by which a comparison is currently made between the load of the prime mover and the total effect produced by the hydraulic pumps, and so that, when the effect of the pumps exceeds the load, the control actuating devices are actuated to reduce the yield of the pumps.

Such comparison and utilization of the result to control the hydraulic pumps can be attained in various different manners,

two of these, considered at the present time the more convenient, are described in the following with reference to FIGS. 3 and 4.

FIG. 3 shows schematically the electronic control system for adjustment of for example, the hydraulic pumps 24A, 24B, and 24C serving to operate the jacks 10A, 10B, and 10Cv In FIG. 3 the pumps as well as the jacks are indicated schematically only, and the hydraulic connections between the pumps and the jacks, shown in FIG. 1 or FIG. 2, are left out for the sake of clarity.

Since the electronic control systems for the individual pumps with appurtenant jacks are similar, only the control system for the jack 10A is described in detail, and, consequently, in the drawing only such parts of the electronic control system connected to the pump 24A and the jack 10A are provided with reference numbers. It should also be noted that the system can, of course, as in fact indicated on the drawing, be connected to any number of pumps and appurtenant jacks.

The direct control can be effected by means of an operating-value-adjustment 210, shown as a potentiometer, the tap 211 of which can be adjusted by a handle 212. The operatingvalue-potentiometer cooperates with an actual-value-potem tiometer 213 having its tap 214 drive-connected to the piston rod 13A of the jack 10A, and so that it is moved proportionally to the movement of the piston rod. The ends of the two potentiometers are interconnected by leads, 215 and 216 respectively, connected one each to the terminals of a voltage source 217, shown as an electric batter with connection to zero or to earth. The voltage source 217 is common to the entire number ofjacks 10.

The two taps 211 and 214 are connected to one input terminal each of an operational amplifier A1, that is an electronic amplifier provided with two input terminals and one output terminal, affording a very strong amplification, and supplying a voltage that, in respect of both sign and magnitude, depends on the voltage difference between the two input terminals. The output terminal of the operational amplifier is connected to earth through two resistors R1 and R2 constituting jointly a voltage divider.

From the connection between the two resistors R1 and R2 a lead is branched out, which through a resistor R3 is connected to one of the input terminals of an operational amplifier A2, the other input terminal of which is connected to earth through a resistor R4. The output terminal of the amplifier A2 is, through a lead 219 connected to the input terminal of a servo-actuator 220 having the output terminal connected to earth. The servo-actuator 220 is shown schematically as a solenoid with an armature 221 that is resiliently urged towards a central position, and that is connected to the control device of the hydraulic pump 24A, indicated by an arrow 222.

It is easily seen that in case the ratio of the resistances to be found on each side of the tap 211 of the operating-valuepotentiometer 210 being equal to the ratio of the two corresponding resistances, into which the tap 214 is dividing the actual-value-potentiometer 213, the two taps 211 and 214 will have equal voltage. 11' then the tap 211 of the operating-value potentiometer 2) is moved upwards, on the drawing, this tap will get an increased voltage, and the tap 214 a reduced voltage and, therefore, the output terminal of the operational amplifier A1 a positive voltage, that is a multiple of the voltage difference between the taps 211 and 214. A part of this voltage, dependent on the magnitudes of the resistors R1 and R2, will through the lead 218 be applied to the one input terminal of the operational amplifier A2, and since the voltage at the other input terminal of this amplifier is zero, the amplifier will emit a positive voltage supplying a current, depending on the magnitude of the voltage, through the solenoid of the servoactuator 220, which provides in turn a movement, depending on the voltage difierence between the taps 211 and 214, in one direction, of the armature 221 and in turn of the con trolling device 222, and in such direction that the piston rod 13A in the jack cylinder will move upwards. As a consequence of the voltage divider R1, R2 and the two resistors R3 and R4 a pre-determined, relatively low voltage difference between the taps 211 and 214 will involve adjustment of the hydraulic pump 24A to full capacity, which involves that adjustment of the adjusting device 212 to a position corresponding to a certain position of the piston rod 13A will make the piston rod move at maximum rate until just before it reaches this position, whereafter the speed will decline gradually until the tap 214 of the actual-value-potentiometer 213 has reached the position corresponding to the adjusted position of the tap 211 of the operation-value-potentiometer 210.

If, on the other hand, the tap 211 is moved downwards, on the drawing, relative to the tap 214 of the actual-value-potem tiometer 213, a negative voltage will result at the output terminal of the operational amplifier Al, whereby the hydraulic pump 24A is adjusted to supply hydraulic fluid in the direction opposite to the direction of fluid obtained by the previously mentioned adjustment of the operating handle 212.

For obtaining overload-protection in connection with the shown electronic control of the hydraulic pumps 25, it is as sumed, in the embodiment shown in FIG. 3, that the hydraulic pumps 24 are powered by a diesel engine provided with a governor of the type, often used at the present time, operating a control rod 225 which, at the nominal number of revolutions for regulating the fuel supply in dependence of the load, moves between positions which, in respect of the right end of the rod, on the drawing, are indicated by positions A and B, the position A corresponding to idling, and the position B to full load. The diesel engine can be overloaded, however, which involves a reduced number of revolutions, and at maximum overloading the control rod 225 reaches the position corresponding to position C. In case of further increased overload the engine will usually stop. Beyond the position C the control rod 225 has another position D, corresponding to starting position.

in the embodiment shown, a housing 226 is placed at the end of the control rod 225, through which housing a displaceable spindle 227 extends, which is provided with a head 228 at the end facing the control rod 225 and outside of the opposite end of the housing 226 with a set collar 229, whereas a compression spring 230, inserted between the head 228 and the housing 226, urges the head 228 as far as possible away from the housing 226.

Inside the housing 226 a potentiometer 231 is arranged, the tap 232 of which is attached to the spindle 227 in such a manner that it is placed at the end of the potentiometer 231 nearest the control rod 225, when the head 228 takes up its extreme position away from the housing 226. The said end of the potentiometer 231 is connected to a lead 233, and the opposite end to a lead 234, a voltage source 235, shown as a battery, being inserted between the said two leads. The tap 232 of the potentiometer 231 is connected to a lead 236.

Each electronic control system shown comprises a photoelectric resistance 237 inserted between the lead 218 and earth, for example of the type having in the dark a resistance of about 10 ohm, which is reduced, however, through exposure to full light to 100 ohm. Light is given to the photoelectric resistance 237 by an incandescent lamp 238, the filament of which is connected to the two said leads 233 and 236.

Normally, in the above said position of the tap 232, there will be no voltage difference between the leads 233 and 236, and, consequently, the incandescent lamp 238 is out, and owing to its very high resistance the photo-electric resistance 237 will not influence the electronic control.

The end of the head 228 facing the control rod 225 is retained, through the set collar 229, in such a position that the control rod 225 will contact the head 228 immediately before it reaches its position B corresponding to full load at nominal number of revolutions. Thus, when the end of the control rod 225 is in position B, the tap 232 is pushed slightly along the coil of the potentiometer 231 to such a position that a voltage difference between the leads 233 and 236 is provided, just adequate to secure that an increase of this voltage difference will make the incandescent lamp 238 light. If now the prime mover is overloaded, the control rod 225 will be moved between the positions B and C, whereby the tap 232 is moved correspondingly along the coil of the potentiometer 231, so that the voltage between the leads 233 and 236 will increase rapidly. Consequently, the light from the incandescent lamp 238 is intensified, and in turn the resistance of the photo-resistance 237 reduced, whereby the output voltage of the amplifier A2 is also reduced, and thus the servo-actuator 220 is brought to reduce the output of the pump 24A. This change develops gradually, but when the control rod 225 approaches the position C, the incandescent lamps will be fighting at full intensity, whereby such a reduction of the voltage between the two input terminals of the amplifier A2 is provided that the servo-actuator 220 will adjust the pump 24A to its neutral position at which the output of the pump is zero. In case of a sudden overloading occurring,the control rod 225 will very quickly move past the position B towards the position C, the lamp 238 will thus very quickly attain full lighting intensity, and, consequently the servo-actuator 220 will function equally quick, and practically instantaneously bring the pump 24A into neutral position. It should be noted that, through suitable selection of the resistors R1 through R4 in the control circuits of the individual jack cylinders, it can be secured that a certain degree of overloading has different effects on the different hydraulic pumps 24. Thus, in connection with a power shovel, the arrangement may be a such that the control circuits for the jack cylinders providing lifting of the shovel are so adjusted that an overloading aflects the corresponding hydraulic pumps essentially more than the other hydraulic pumps. The same can be attained even when uniform control circuits are used by insertion of difierent filters between the incandescent lamps 238 and the photo-electric resistances 237.

It should also be mentioned that the potentiometer 231 has the part of its windings along which the tap 232 is moving when the control rod 225 is moved between the positions C and D, short-circuited, so that the incandescent lamp 237 will be lighting at full intensity also during the starting of the diesel engine.

It is mentioned above that the control rod 225 is longitudinally displaceable. In just as many cases, however, the control rod is rotatable, in which cases it is most appropriate to connect it to an annular potentiometer, providing the same ef' feet as described above in connection with the use of a slide; potentiometer. I

An electronically functioning control system for the hydraulic pumps 24 provided with another overload-protector for the prime mover is shown in FIG. 4. The control circuits for the individual hydraulic pumps 24, comprising an operationvalue-adjustment and an actual-value adjustment, is the same as shown in FIG. 3, wherefore, in respect of these components, the references used are the same as those in FIG. 3, likewise as these components shall not be explained in more detail. The resistor R5 inserted between the resistor R3 and the output terminal of the operational amplifier is a feedback resistor.

in this embodiment the shah 27 of the prime mover, or some other shaft of the engine, is provided with a four-poled star wheel 240, opposite to which a magnetic pick-up 241 is stationarily arranged, emitting a pulse whenever the stray field of the magnet of the pick-up is changed through the effect of the poles of the star wheel 240, which is made of steel. Hereby sawtooth-pulses are provided, triggering a monostable multivibrator 242 providing a square voltage of constant height and length. This square voltage is applied to a meanvaluemeter 243 producing a DC. voltage proportional to the mean value of the square voltage, with a suitable time-constant. Thus, this voltage will be proportional to the number of revolutions of the engine, and within a wide field the effect produced by the engine will likewise be proportional to the number of revolutions. A closer approximation of the voltage to the effect produced by the engine can be obtained, how ever, through insertion after the mean value meter 243 of a resistance-diode-matrix, whereby it is possible to make the mean value voltage follow a curve fairly close corresponding over a still wider field to the dependency of the engine effect on the number of revolutions. The mean value voltage thus attained is through a resistor R6 applied to one of the input terminals of an operational amplifier A3, functioning as a differential amplifier, this input terminal being, moreover, connected to earth through a resistor R7.

The effect produced by each individual hydraulic pump, or transmitted where it functions as a motor, is the product of the amount of fluid given ofi by the pump per unit of time and the pressure diflerence between suction and pressure side of the pump. The voltage applied by the output terminal of the operational amplifier A2 to the coil of the servo-actuator 220 is proportional to the amount of fluid given off. With a view to attaining an electric voltage corresponding to the said pressure difierence, each pump 24 is connected to a pressuredif ference transducer, the ports 25 and 26 of the pump being connected to one end each of a pressure-difference indicator 244, which is connected to the adjustable tap 245 of a potentiometer 246, and in such a manner that zero pressure difference between the two ports 25 and 26 makes the tap 245 take up its central position on the potentiometer 246.

To attain multiplication of the voltage indicating the output of the hydraulic pump, and of the voltage indicating the pressure difference between the two ports of the pump, a differential amplifier 247 is used. This amplifier 247 is energized from a DC. source 248, the negative voltage thereof being ap' plied to a lead 249, and the positive voltage to a lead 250. The difl'erential amplifier 247 is provided with two NPN transistors T1 and T2, the emitters thereof being, through equal resistors R8 and R9, connected to a lead 251, which through a resistor R10 is connected to the positive lead 250. The collectors of the transistors are connected, through equal resistors RI! and R12, to the negative lead 249. The resistance of the resistor R10 exceeds considerably the sum of the resistance of the resistors R8 and R1 or R9 and R12, ensuring that the current passing through the resistor R10 will be approximately constant. The potentiometer of the transducer 244, 245, 246 is inserted between the collectors of the transistors T1 and T2. To the base of the transistor T1 is applied through a resistor R13 the voltage given ofl by the operational amplifier A2, whereas the base of the transistor T2 through a resistor R14 of the same resistance as the resistor R13 is connected to earth.

When the hydraulic pump 24 is in neutral position, that is when the voltage given ofi by the operational amplifier A2 is zero, and the tap 245 of the transducer 244, 245, 246 is in its central position, there will be no voltage difference between the collectors of the two transistors T1 and T2, thus having equal constant voltage. When a positive voltage is found at the output terminal of the operational amplifier A2, less current will pass the transistor T1, and correspondingly more will pass the transistor T2, since the total of the currents through these two transistors remains constant. The voltage of the collector of the transistor T1 will, therefore, be increasingly negative, and the voltage of the collector of the transistor T2 increasingly positive, but the voltage at the middle of the potentiometer coil 246 will always be of the aforesaid constant value. The current passing through the coil of the potentiometer 246, from the collector of the transistor T2 to the collector of the transistor T1, will be proportional to the voltage given ofl by the operational amplifier A2, the signs being taken into account. This involves that, when the pump 24 is functioning and there is a pressure difference over its ports, the tap 245 is moved away from the middle-point of the potentiometer 246. Therefore, the voltage at the tap 245 will be proportional to the voltage given ofl by the operational amplifier A2 as well as to the distance of the potentiometer tap 245 from the middleposition plus the constant voltage.

If, on the other hand, the load on the hydraulic pump 24 is a such that the pump functions as a motor and supplies energy to the prime mover, the tap 245 will be moved to the opposite side of the middle-position and give off a voltage of opposite sign.

The voltage at the tap 245 is through a resistor R trans mitted to an interconnecting lead 252, whereby the taps 245 of all of the transducers of the hydraulic pumps 24 are interconnected in a corresponding manner. The lead 252 is connected to one input terminal of an operational amplifier A4, the other input terminal of this amplifier being connected to a voltage divider consisting of two resistors R16 and R17, connected to the two leads 249 and 250. Through this voltage divider the aforesaid constant voltage is balanced. This involves that the the voltage given off from the output terminal of the operational amplifier A4 is proportional to the sum of the effects of the hydraulic pumps, with signs. The outlet terminal of the amplifier A4, being through a feedback resistance R18 connected to the input terminal first mentioned, is, through a lead 253 connected to earth through a resistor R19 as well as to the other input terminal of the operational amplifier A3 through a resistor R20. This operational amplifier undertakes a comparison of the effect produced by the prime mover, that is the load of the prime mover, and the total aggregate effect produced by the hydraulic pumps 24, and the amplifier A3 will emit a voltage proportional to the difference of the two effects, with signs, in such a manner that it is positive, when the effect produced by the hydraulic pumps is higher than corresponding to the load of the prime mover. The output terminal of the operational amplifier A3 is through a rectifier 254, for example a diode, connected to one, 233, of the two leads connected to the incandescent lamps, whereas the other, 236, of these leads is connected to earth. The rectifier 254 ensures that no current will pass to the lead 233 unless the pumpeffect exceeds the load of the prime mover, but in such case the lamps 238 will be lighting and affect the photo-electric resistances, the result being that the effect of the hydraulic pumps 24 is reduced.

As the control shall not be effective until immediately before the horse power-curve of the prime mover is exceeded, and as the control shall be fully effective at a slight overload, the operational amplifier A3 must be adapted to produce full voltage at a slight difference between the input voltages, that is it must be over-excited for a voltage of a few per cent of the maximum input voltage.

ln the examples given above, an incandescent lamp 238 is placed opposite each photo-electric resistance 23'], but nothing prevents, of course, that all, or a number of the photoelectric resistances 237 being so placed that exposure can be efiected by means of one single incandescent lamp 238.

It should also be noted that a voltage proportional to the effect produced by the hydraulic pumps 24 can be obtained in other way than by use of pressure transducers 244, 245, 246, as aforementioned. Thus a simple pressure transducer may be connected to each pump port, for example so that each of the potentiometer coils of the two sets of transducers is inserted between a branch-out from the output terminal of the corresponding operational amplifier A2 and earth, and the two branch-outs are connected to one input terminal each of an operational amplifier, which may be a joint device for the circuits belonging to the individual hydraulic pumps, and which may have its output terminal connected to a differential amplifier corresponding to the amplifier A3 in FIG. 4.

What I claim is:

l. A hYdraulic system comprising in combination a plurality of mutually independent double acting hydraulic motors;

each said hydraulic motor being provided with an inlet for each direction of movement;

for each said hydraulic motor a main control device adapted to control and reverse said hydraulic motor independently of the remaining hydraulic motors;

a pump unit powered by a prime mover for providing the required hydraulic effect; said pump unit comprising for each said hydraulic motor an adjustable hydraulic pump having two ports and adapted solely to pressurize fluid to drive said hydraulic motor; each said pump including a pump control device; a first hydraulic connection between one of said two ports of said pump and one of said inlets of said hydraulic motor; a second hydraulic connection between the second one of said two ports of said pump and the second one of said two inlets of said hydraulic motor; said pump being of a type having for each adjustment a constant voluminal yield; said main control device of said hydraulic motor being connected to said pump control device for adjustment thereof; said pump further being of a type adjustable by means of said main control device through said pump control device from a position,

2. A hydraulic system as defined in claim 1, at least one of said hydraulic connections comprising a pilot controlled valve controlled by the pressure in the other one of said two hydraulic connections in such a manner that its flow area increases with increasing pressure of said other one of said two hydraulic connections.

3. A hydraulic system as defined in claim 1, at least one said hydraulic connection comprising a main pipe interconnecting said inlet and said port, said main pipe comprising a discharge valve preventing flow of liquid from said hydraulic motor to said pump, a shunt pipe bridging said discharge valve, a pilot controlled throttle valve inserted in said shunt and controlled by the pressure in the other one of said two hydraulic connections, and so that it is maintained closed at a pressure below a certain minimum value in said second one of said two hydraulic connections, whereas its flow area increases with increasing pressure in the last said hydraulic connection.

4. A hydraulic system as defined in claim 3, and comprising a reservoir for hydraulic fluid; said shunt pipe communicating with said reservoir; said main pipe comprising a pipe portion interconnecting said one of said two ports of said pump and said discharge valve; said shunt pipe comprising a shunt portion connected to said pipe portion, said shunt portion comprising a suction valve adapted to open through suction from said pump.

5. A hydraulic system as defined in claim 1, comprising first means for measuring the load of said prime mover, second means for measuring the total output of said hydraulic pumps, third means for comparison of the said load and the said output, and control-actuating devices adapted to provide actua tion of the said pump control devices For said hydraulic pumps, whenever said total output exceeds the output corresponding to full load of said prime mover.

6. A hydraulic system as defined in claim 5, said control actuating devices comprise a unit adapted to emit a warning signal.

7. A hydraulic system as defined in claim 5, said control actuating devices being adapted to actuate said pump control devices to control the effect produced by individual ones of said hydraulic pumps.

8. A hydraulic system as defined in claim 5, said prime mover being an internal combustion engine, said engine being equipped with a governor responsive to the load on said engine and comprising a moveable fuel control rod able to take up a first position corresponding to idling without load, a second position corresponding to full load at maximum number of revolutions, and a third position corresponding to full load at minimum number of revolutions, that is maximum overload; said pump control device of each said hydraulic pumps being connected to and adapted to be adjusted by a corresponding electrically controlled servo-actuator, an electric safety circuit; coupling means coupling said servo-actuators to said safety circuit; said safety circuit being fed through an adjustable electric resistance, for example a potentiometer, provided with a slidable tap; said control rod being adapted during its movement between its said second position and its said third position to operate said slidable tap thereby changing the voltage over said safety circuit for through said coupling means amending the adjustment of said servo-actuators, so that the yield of the said hydraulic pumps controlled through said servoactuators is reduced during said movement of said control rod.

9. A hydraulic system as defined in claim 8, said safety circuit including at least one electric incandescent lamp, preferably one for each said servo-actuator; each said servoactuator having a control circuit; said control circuit includinG as said coupling means a light-sensitive electric resistance exposed to the light from said incandescent lamp, or one of said lamps; said light-sensitive electric resistance being arranged in said control circuit, preferably between a connection thereof, affecting the feed current of said servo-actuator, and zero, in such a manner that increase of the light intensity involves such an adjustment of the said servo-actuator that a reduction of the yield of the corresponding one of the said hydraulic pumps results.

10. A hydraulic system as defined in claim 5, each said pump control device being connected to and controlled by a corresponding electric servo-actuator; an electronic measuring device connected to said prime mover and adapted to emit a pilot voltage substantially proportional to the load on said prime mover; a voltage difference indicator, for example an operation amplifier, said pilot voltage being applied to the one input of said first voltage difi'erence indicator; each said servoactuator having a control circuit, means for tapping from said control circuit a second voltage proportional to the feed current of said servo-actuator; a difl'erential amplifier; said second voltage being applied to the one input terminal of said difl'erential amplifier, the second input terminal of said differential amplifier being applied with a constant voltage, preferably zero voltage; a pressure-difierence transducer inserted between said two ports of each said hydraulic pump, said pressure-difference transducer comprising a control potentiometer having a slidable tap, the position of which depends on the pressure-difference between said two ports; the two ends of said control potentiometer being connected to one of the two output terminals of said differential amplifier each; summing up means for electrically summing up the voltages of said taps to provide a summed up voltage; said summed up voltage being applied, preferably through an amplifier, to the second input terminal of said voltage difference indicator, a rectifier permitting flow in one direction only; a safety circuit fed through said rectifier from the output terminal of said voltage difference indicator; each said control circuit containing an actuator control device; said safety circuit containing control actuating means adapted to actuate said actuator control devices in such a manner that the yield of the said hydraulic pump belonging to each said servo-actuator is reduced in dependency of the voltage applied to said safety circuit.

11. A hydraulic system as defined in claim 10, said control actuating means being one or more incandescent lamps; said actuator control device being a light-sensitive electric resistance exposed to the light from said incandescent lamp, or

one of said lamps, and arranged in said control circuit in such a manner, preferably between a connection thereof, affecting the feed current of said servo-actuator, and zero, that increase of the light intensity involves such an adjustment of said servoactuator that a reduction of the yield of the corresponding one of said hydraulic pumps results.

i I i t 

1. A hYdraulic system comprising in combination a plurality of mutually independent double acting hydraulic motors; each said hydraulic motor being provided with an inlet for each direction of movement; for each said hydraulic motor a main control device adapted to control and reverse said hydraulic motor independently of the remaining hydraulic motors; a pump unit powered by a prime mover for providing the required hydraulic effect; said pump unit comprising for each said hydraulic motor an adjustable hydraulic pump having two ports and adapted solely to pressurize fluid to drive said hydraulic motor; each said pump including a pump control device; a first hydraulic connection between one of said two ports of said pump and one of said inlets of said hydraulic motor; a second hydraulic connection between the second one of said two ports of said pump and the second one of said two inlets of said hydraulic motor; said pump being of a type having for each adjustment a constant voluminal yield; said main control device of said hydraulic motor being connected to said pump control device for adjustment thereof; said pump further being of a type adjustable by means of said main control device through said pump control device from a position,
 2. A hydraulic system as defined in claim 1, at least one of said hydraulic connections comprising a pilot controlled valve controlled by the pressure in the other one of said two hydraulic connections in such a manner that its flow area increases with increasing pressure of said other one of said two hydraulic connections.
 3. A hydraulic system as defined in claim 1, at least one said hydraulic connection comprising a main pipe interconnecting said inlet and said port, said main pipe comprising a discharge valve preventing flow of liquid from said hydraulic motor to said pump, a shunt pipe bridging said discharge valve, a pilot controlled throttle valve inserted in said shunt and controlled by the pressure in the other one of said two hydraulic connections, and so that it is maintained closed at a pressure below a certain minimum value in said second one of said two hydraulic connections, whereas its flow area increases with increasing pressure in the last said hydraulic connection.
 4. A hydraulic system as defined in claim 3, and comprising a reservoir for hydraulic fluid; said shunt pipe communicating with said reservoir; said main pipe comprising a pipe portion interconnecting said one of said two ports of said pump and said discharge valve; said shunt pipe comprising a shunt portion connected to said pipe portion, said shunt portion comprising a suction valve adapted to open through suction from said pump.
 5. A hydraulic system as defined in claim 1, comprising first means for measuring the load of said prime mover, second means for measuring the total output of said hydraulic pumps, third means for comparison of the said load and the said output, and control-actuating devices adapted to provide actuation of the said pump control devices For said hydraulic pumps, whenever said total output exceeds the output corresponding to full load of said prime mover.
 6. A hydraulic system as defined in claim 5, said control actuating devices comprise a unit adapted to emit a warning signal.
 7. A hydraulic system as defined in claim 5, said control actuating devices being adapted to actuate said pump control devices to control the effect produced by individual ones of said hydraulic pumps.
 8. A hydraulic system as defined in claim 5, said prime mover being an internal combustion engine, said engine being equipped with a governor responsive to the load on said engine and comprising a moveable fuel control rod able to take up a first position corresponding to idling without load, a second position corresponding to full load at maximum number of revolutions, and a third position corresponding to full load at minimum number of revolutions, that is maximum overload; said pump control device of each said hydraulic pumps being connected to and adapted to be adjusted by a corresponding electrically controlled servo-actuator; an electric safety circuit; coupling means coupling said servo-actuators to said safety circuit; said safety circuit being fed through an adjustable electric resistance, for example a potentiometer, provided with a slidable tap; said control rod being adapted during its movement between its said second position and its said third position to operate said slidable tap thereby changing the voltage over said safety circuit for through said coupling means amending the adjustment of said servo-actuators, so that the yield of the said hydraulic pumps controlled through said servo-actuators is reduced during said movement of said control rod.
 9. A hydraulic system as defined in claim 8, said safety circuit including at least one electric incandescent lamp, preferably one for each said servo-actuator; each said servo-actuator having a control circuit; said control circuit includinG as said coupling means a light-sensitive electric resistance exposed to the light from said incandescent lamp, or one of said lamps; said light-sensitive electric resistance being arranged in said control circuit, preferably between a connection thereof, affecting the feed current of said servo-actuator, and zero, in such a manner that increase of the light intensity involves such an adjustment of the said servo-actuator that a reduction of the yield of the corresponding one of the said hydraulic pumps results.
 10. A hydraulic system as defined in claim 5, each said pump control device being connected to and controlled by a corresponding electric servo-actuator; an electronic measuring device connected to said prime mover and adapted to emit a pilot voltage substantially proportional to the load on said prime mover; a voltage difference indicator, for example an operation amplifier, said pilot voltage being applied to the one input of said first voltage difference indicator; each said servo-actuator having a control circuit, means for tapping from said control circuit a second voltage proportional to the feed current of said servo-actuator; a differential amplifier; said second voltage being applied to the one input terminal of said differential amplifier, the second input terminal of said differential amplifier being applied with a constant voltage, preferably zero voltage; a pressure-difference transducer inserted between said two ports of each said hydraulic pump; said pressure-difference transducer comprising a control potentiometer having a slidable tap, the position of which depends on the pressure-difference between said two ports; the two ends of said control potentiometer being connected to one of the two output terminals of said differential amplifier each; summing up means for electrically summing up the voltages of said taps to provide a summed up voltage; said summed up voltage being applied, preferably through an amplifier, to the second input terminal of said voltage difference indicator, a rectifier permittIng flow in one direction only; a safety circuit fed through said rectifier from the output terminal of said voltage difference indicator; each said control circuit containing an actuator control device; said safety circuit containing control actuating means adapted to actuate said actuator control devices in such a manneR that the yield of the said hydraulic pump belonging to each said servo-actuator is reduced in dependency of the voltage applied to said safety circuit.
 11. A hydraulic system as defined in claim 10, said control actuating means being one or more incandescent lamps; said actuator control device being a light-sensitive electric resistance exposed to the light from said incandescent lamp, or one of said lamps, and arranged in said control circuit in such a manner, preferably between a connection thereof, affecting the feed current of said servo-actuator, and zero, that increase of the light intensity involves such an adjustment of said servo-actuator that a reduction of the yield of the corresponding one of said hydraulic pumps results. 